The present invention relates generally to wireless communication systems, and more particularly, to power management for multi-carrier wireless broadband terminals.
In a multi-carrier wireless communication system, a transmit signal has many sub-carriers or tones. Each sub-carrier or tone carries a pre-determined amount of information in a pre-defined bandwidth. For instance, in certain multi-carrier system, a downlink transmit signal from a base station (BTS) transmitter consists of ten 500 kHz sub-carriers to form a 5 MHz wide signals.
In licensed bands, such as MMDS band (2500-2686 MHz) and WCS band (2305-2360 MHz), in order to minimize interferences from one band to another, Federal emission rules limit the amount of out-of-band spurious emissions power radiated out of a transmitter. As a result, a transmitter output power is limited to a maximum allowable level that ensures the compliance to the regulatory rules.
The out-of-band emissions that are near the band edge are commonly caused by the intermodulation distortions from the transmitter. The out-of-band emissions that are far away from the band edge are commonly caused by the noise floor of the transmitter or the combination of the noise floor and intermodulation distortions of the transmitter.
Intermodulation distortion is nonlinear distortion characterized by the appearance, in the output of a device, of frequencies that are linear combinations of the fundamental frequencies and all harmonics present in the input signals.
In order to meet the FCC regulations, equipment providers commonly implement technologies as required, which are summarized below.
A first technology is a high power linear amplifier to minimize the intermodulation distortions. Its advantages are higher transmit power, better system link budget. Yet high power linear amplifier has disadvantages, such as high cost, high power consumption and large size (due to heat sink). Therefore, high power linear amplifier is suitable for the BTS implementation, and not for Customer-premises-equipment (CPE) terminals that prefer smaller size and lower cost.
A second technology is a channel filter to filter out the intermodulation distortions and to reduce the out-of-band noise floor. Its advantage is lower out-of-band spurious emissions. But its disadvantages are high cost due to added filter, lower transmitter power due to filter loss, fixed frequency channel and inability to reduce the out-of-band spurious emissions that are near the band edge. Therefore, channel filter is also suitable for the implementation on BTS and not for CPE terminals that require abilities to communicate with different BTS with different frequency channels.
A third technology is to provide extra guard band to a band edge, which allows higher transmit power, without implementing high cost liner amplifier. But its disadvantages are inefficient use of spectrum, reduced capacity and higher overall system cost.
Therefore, it is desirable to devise an improved method for use in a wireless broadband CPE terminal such as, PCMCIA cards, to meet the government emission rules, yet still maintains low cost, low power consumption and reasonable size.